1. Field of the Invention
The present invention relates to printers such as serial printers, and more particularly to the control of such printers.
2. Description of the Related Art
Known serial printers include a print head for printing characters on a print form or paper, a carriage portion for loading thereon the print head, a space motor for moving the carriage portion, a line feed motor for feeding the print form, and a control unit for controlling operation of the aforesaid component parts. The control unit is connected to the space motor, the line feed motor and the print head via connecting cords. A typical example of such known serial printers will be described below with reference to the accompanying drawings.
FIG. 6 shows the general construction of the conventional serial printer which includes a control unit 1 and a carriage portion 2 connected together via a connecting cord 3. The control unit 1 has a control portion 4 and a drive circuit 5. The control portion 4 is composed of a microprocessor, for example, and serves to control a space motor and a line feed motor (neither shown) as well as the drive circuit 5. The carriage portion 2 includes a print head 8 and a cooperating pair of head connectors 6, 7 for connection of the print head 8, and is movable upon rotation of the space motor. The head connector 6 is disposed on a support plate (not shown) of the carriage portion 2 and is connected with the connecting cord 3. The head connector 7 is provided on the print head 8 and adapted to be coupled with the head connector 6 for connecting the input side of the print head 8 through the connecting cord 3 to the output side of the drive circuit 5.
FIG. 7 shows a schematic circuit diagram of a conventional matrix printer used for driving a print head 8a consisting of 24 pins or wires. In this figure, those component parts designated by reference characters 3a-8a correspond respectively to the component parts 3-8 shown in FIG. 6 and the suffix "a" indicates that the respective component parts are so constructed as to meet the various requirements for driving the print heads 8a consisting of 24 pins. The print head 8a includes a total of 24 driving coils (electromagnets) 801-824 each for selectively driving a corresponding one of the 24 pins arranged in two columns of 12 pins which correspond in position to two rows of 12 dots to be mutually printed. The control portion 4a delivers to the drive circuit 5a, two timing signals T.sub.1, T.sub.2 for determining the timing of the application of driving power to the respective columns of 12 pins, and drive signals D.sub.1 -D.sub.24 for driving the respective pins. The drive circuit 5a includes two PNP transistors 91, 92 having emitters connected with the power supply and adapted to be turned on and off by the timing signals T.sub.1, T.sub.2, and a total of 24 NPN transistors 101-124 having emitters connected to the ground and adapted to be turned on and off by the drive signals D.sub.1 -D.sub.24. Collectors of the transistors 91, 92 are connected successively through the connecting cord 3a and the head connectors 6a, 7a with one end of the drive coils 801-812 and one end of the drive coils 813-824, respectively. Collectors of the transistors 101-124 are connected successively through the connecting cord 3a and the head connectors 6a, 7a with the other end of the drive coils 801-824, respectively. Thus, each of the head connectors 6a, 7a has at least 26 pins or terminals.
Operation of the printer thus constructed is described below.
When the level of timing signals T.sub.1, T.sub.2 delivered from the control portion 4a is low (L), the transistors 91, 92 are turned on whereupon a drive voltage is supplied from the drive power supply to one end of the drive coils 801-812 and also to one end of the drive coils 813-824. When drive signals D.sub.1 -D.sub.24 delivered from the control portion 4a are high (H), the transistors 101-124 are turned on, whereupon the other end of the drive coils 801-824 becomes low (L). Consequently, an electric current flows through those drive coils to which the drive voltage is applied, thereby driving corresponding pins of the print head 8a. Thus, the 24 pins of the print head 8a are selectively driven depending on the level (H/L) of the timing signals T.sub.1, T.sub.2 and the drive signals D.sub.1 -D.sub.24.
FIG. 8 shows a schematic circuit diagram of another conventional printer used for driving a print head 8b consisting of 9 pins. In this figure, those component parts designated by reference characters 3b-8b correspond respectively to the component parts 3-8 shown in FIG. 6 and the suffix "b" indicates that the respective component parts are so constructed to satisfy the requirements for driving the print head 8b consisting of 9 pins. The print head 8b includes a total of 9 drive coils 801-809 each for selectively driving a corresponding one of the 9 pins corresponding in position to a column or row of 9 dots to be mutually printed. The control portion 4b delivers to the drive circuit 5b a timing signal T.sub.1, and a total of 9 NPN transistors 101-109 having emitters connected to the ground and adapted to be turned on and off in response to the drive signals D.sub.1 -D.sub.9. The collector of the transistor 91 is connected through the connector cord 3b and the head connectors 6b, 7b with one end of the drive coils 801-809, the other end of the drive coils 801-809 being connected in common with collectors of the respective transistors 101-109.
The printer of the foregoing construction operates as described below.
When a timing signal T.sub.1 delivered from the control portion 4b is low (L), the transistor 91 is turned on whereupon the drive power supply applies a drive voltage to one end of the drive coils 801-809. In this instance, if drive signal D.sub.1 -D.sub.9 are high (H), the transistors 101-109 turn on, thereby enabling an electric current to flow through the corresponding drive coils 801-809. Thus, the 9 pins of the print head 8b are selectively driven in dependence on the level (H/L) of the timing signal T.sub.1 and the drive signals D.sub.1 -D.sub.9.
The conventional printers described above are not fully satisfactory because of the following reasons.
The printer incorporating the 24-pin print head 8a shown in FIG. 7 is advantageous in that the print quality is high because the diameter of each individual pins is small and the printed character is composed of a large number of dots. On the other hand, the printer of this type has drawbacks in that the print head 8a is heavy and hence gives an increased load on the space motor for driving the carriage portion. With this increased load, the printer is not suitable for a high speed printing operation and consumes a considerable amount of electric power.
Conversely, the printer having the 9-pin print head 8b shown in FIG. 8 is disadvantageous in that due to a large diameter of each individual pin and a small number of dots constituting the character, the print quality is low. The printer of this type is, however, lightweight and hence is suitable for a high speed printing operation and consumes only a small amount of electric power.
Since a high speed and high quality printing can be obtained only by an expensive and large-sized printer, it has been customary practice to select for use the 24-pin print head printer and the 9-pin print head printer in view of the desired purposes, and in consideration of the advantages of the respective printers. Such a conventional practice is however tedious and time-consuming and requires two printers which are costly to maintain.